1. Field of the Invention
This invention relates to a method and an apparatus for producing a thermoplastic resin sheet, in which the obtained polypropylene resin sheet or film can be used for the packaging of food, medicine and so on.
2. Description of the Related Art
Of late years, as a transparentizing art of sheet (or film) made out of polypropylene resin, various processes with the use of a metallic endless belt have been proposed.
The greatest advantage of the use of the metallic endless belt is the point that high-glossing of both faces of the sheet can be effectively obtained to continuously transfer a mirror face of the metallic endless belt onto the surface of the sheet.
In order to increase the transparency of the polypropylene resin sheet, however, in addition to the aforementioned high-glossing, it is required that haze (the degree of cloud) inside the sheet is reduced to a low level.
Conventionally, various methods and apparatus for improving the transparency have been proposed.
For example, according to Japanese Patent Application Laid-open No. 6-170919, an apparatus for cooling and solidifying a fused sheet is disclosed, in which the metallic endless belt is provided to be wound on and between a metal roller and an elastic roller, covered with an elastic body on its surface, and after the sheet is guided into the area between the elastic roller and the cooling roller through the metallic endless belt, the sheet is, continuously, cooled while being pressed to a part of the circumference of the cooling roller.
In this case of the apparatus, the effect of immediately cooling the sheet pressed by the cooling roller and the metallic endless belt is insufficient, and in addition, hardness of the elastic body of the elastic roller is higher, so that the sheet is not touch-rolled on its face, with the result that a resin bank is produced or a fine transparency of the sheet cannot be obtained.
In Japanese Patent Application Laid-open No. 6-166089 and Japanese Patent Application Laid-open No. 6-170919, a method for cooling the sheet using the same method after the metallic endless belt is provided to be wound on and between the metal roller and the elastic roller covered with the elastic body on its surface is disclosed, however, the cooling temperature is higher and hardness of the elastic body is higher, so that the immediate cooling effect is insufficient, with the result that the transparent polypropylene sheet cannot be obtained without the use of raw materials consisting of a nucleating agent.
Further, according to Japanese Patent Application Laid-open No. 6-55613, a producing method of a thermoplastic resin sheet is disclosed, in which the sheet of the fusing state, consisting of a nucleating agent or the like, is guided into an area between the cooling roller and the metallic endless belt, and after the sheet is moved while being touched to the cooling roller and the metallic endless belt, the sheet is pressed against the cooling roller from the inside of the metallic endless belt by the aforementioned cooling roller.
In this case of the producing method, the immediate cooling of the sheet starts after the sheet is sandwiched between the cooling roller and the metallic endless belt, so that at the starting point of the immediate cooling, the face pressure is insufficient, with the result that it is difficult to obtain a fine effect of the immediate cooling and a fine surface gloss on the sheet. As a result, the transparency has tried to be improved by adding a nucleating agent or the like, but the degree of transparency may be insufficient if the nucleating agent or the like is not added.